Expectations for a light-power technology and a power semiconductor including a solar cell that directly converts solar light energy into electric energy have rapidly grown in recent years as a next generation energy source especially from the viewpoint of the global environmental problem. There are various types of solar cells, such as one using a compound semiconductor or an organic material. However, one using a silicon crystal is the mainstream at present. In the solar cell that is currently produced and sold most commonly at present, an n electrode is provided on a photo-detecting surface that detects sunlight, and a p electrode is provided on the backside. The n electrode provided on the photo-detecting surface side is indispensable for taking out a current. However, because no sunlight enters the substrate under this electrode, power generation does not occur in this part. Therefore, conversion efficiency decreases when the area of the electrode is large. A loss due to the electrode on the photo-detecting surface side in such way is referred to as a shadow loss.
Because the backside electrode type solar cell having no electrode on the photo-detecting surface does not suffer from a shadow loss due to the electrode and can take 100% of the incident sunlight into the solar cell, high efficiency can be theoretically realized. In Patent Document 1 (U.S. Pat. No. 4,927,770), one embodiment of the backside electrode type solar cell is disclosed that is suitable for a light-condensing type. An outline of its structure is shown in FIG. 1.
In the backside electrode type solar cell, a high-concentration p-doped region 12 and a high-concentration n-doped region 13 are provided alternately on the backside of a semiconductor substrate 10. Further, in the backside electrode type solar cell, a texture etching surface 18 is formed on the photo-detecting surface of semiconductor substrate 10. A passivation layer 11 is formed on the surface of semiconductor substrate 10, and with this, surface re-bonding is suppressed. A p electrode 14 is connected to high-concentration p-doped region 12 and an n electrode 15 is connected to high-concentration n-doped region 13 respectively through a p region contact hole 16 and an n region contact hole 17 provided on the backside, and a current is taken out from these. Passivation layer 11 on the photo-detecting surface works as a reflection preventive film also. As seen in FIG. 1, all of the high-concentration p and n-doped regions and the electrodes are formed on the backside, and because there is nothing to block the light on the surface (photo-detecting surface), 100% of the sunlight can be taken in.
The backside electrode structure of the backside electrode type solar cell has a fine pattern in which high-concentration n-doped region 13 and high-concentration p-doped region 12 are formed alternately. A manufacturing process of the solar cell is described in the following using FIG. 1.
First, passivation layer 11 is formed by forming an oxide film on the surface of semiconductor substrate 10 and then depositing a nitride film on the oxide film. Next, using a photolithography technique, n region contact hole 17 is opened in passivation film 11 and a glass layer containing an n-type dopant is deposited on the surface of semiconductor substrate 10 by CVD (a chemical vapor deposition method). A part of the glass layer that corresponds to the high-concentration p-doped region is removed, p region contact hole 16 is formed in passivation film 11 by a photolithography technique, and a glass layer containing a p-type dopant is deposited on the surface of semiconductor substrate 10. When this semiconductor substrate 10 is heated at 900° C., high-concentration p-doped region 12 and high-concentration n-doped region 13 are formed on the backside. After that, the entire glass layer remaining on the surface of semiconductor substrate 10 is removed, a thermal process is performed at a high temperature of 900° C. or more, and a hydrogenated process is performed on the interface between Si—SiO2 in H2. After removing two glass layers that became a diffusion source, p electrode 14 and n electrode 15 are formed by depositing Al on the backside of semiconductor substrate 10 by sputtering or the like, and patterning by a photolithography technique.
Further, as to a semiconductor doping technique, researches of a dopant paste that can screen-print to form p, p+, n, and n+ regions on a single crystal and polycrystalline silicon substrate less expensively and easily and a masking paste that controls dopant diffusion in semiconductor manufacturing have been carried out (refer to Patent Document 2 (Japanese National Patent Publication No. 2002-539615)).
Further, researches related to a technique of performing patterning with a mask for controlling diffusion of a dopant in advance and then performing doping in order to overcome a decrease in a minority carrier lifetime of the semiconductor substrate in the manufacturing process of the solar cell have been carried out (refer to Patent Document 3 (Japanese Patent Laying-Open No. 2002-124692)).
Patent Document 1: U.S. Pat. No. 4,927,770
Patent Document 2: Japanese National Patent Publication No. 2002-539615
Patent Document 3: Japanese Patent Laying-Open No. 2002-124692